System and method for throughput enhancement

ABSTRACT

A method in a first node of a network comprises computing a packet duration when using a first unicast profile with a first type preamble and a packet duration when using a second unicast profile with a second type preamble; comparing the computed packet durations so as to determine one from the first and second unicast profiles which yields a shorter duration; and sending the packet to a second node by using the determined unicast profile.

PRIORITY REFERENCE TO PRIOR APPLICATION

This application claims benefit of and incorporates by reference U.S.patent application Ser. No. 61/377,091, entitled “Throughput Enhancementand Modulation Profile Memory Size Reduction,” filed on Aug. 26, 2010,by inventors Yehuda Azenkot et al.

TECHNICAL FIELD

This invention relates generally to receivers and more particularly, butnot exclusively, provides a method and apparatus for throughputenhancement in a Multimedia Over Coax Alliance (MoCA) system via the useof multiple unicast profiles.

BACKGROUND

The MoCA 2.0 standard defines Modulation Profiles that have a set ofparameters that determines the transmission between nodes, includingpreamble type, cyclic prefix length, modulations per subcarrier andtransmit power. Each MoCA 2.0 Node maintains two PHY profiles for each100 MHz unicast link: one for Nominal Packet Error Rate of 1e-6 (NPER)and one for Very Low Packet Error Rate of 1e-8 (VLPER). Similarly, twoPHY profiles are maintained for 100 MHz Greatest Common Density (GCD) orBroadcast profiles: one for NPER (1e-6) and one for VLPER (1e-8). TheGCD is a modulation format computed by a node for transmission tomultiple recipient nodes. For the GCD or Broadcast format, themodulation used for each subcarrier is chosen to be the greatestpossible modulation density that is less than or equal to the modulationdensity for that subcarrier as reported in the most recent Error VectorMagnitude (EVM) Probe Report the node sent to each of the other nodes.The EVM probe is used to determine the optimum modulation scheme for aset of subcarriers. During a new Node admission procedure, PHY profilesfor both NPER and VLPER are distributed. Since the VLPER packets aremore robust to errors than the NPER packets, the modulations persubcarrier of the VLPER profile are usually equal or lower than thecorresponding modulations per subcarrier of the NPER profile.

However, the MoCA 2.0 standard does not support the use of two separateunicast high-speed profiles for each of NPER and VLPER packets. For eachtype of profile, either a Type5 preamble or a Type 6 preamble can beused, but not both.

Accordingly, a system and method to increase throughput via the use ofmultiple profiles may be desirable.

SUMMARY

Embodiments of the invention provide a system (node) and method forincreasing throughput.

Specifically, according to an embodiment of the invention, a method in afirst node of a network comprises computing a packet duration when usinga first unicast profile with a first type preamble and a packet durationwhen using a second unicast profile with a second type preamble;comparing the computed packet durations so as to determine one from thefirst and second unicast profiles which yields a shorter duration; andsending the packet to a second node by using the determined unicastprofile.

According to an embodiment of the invention, a method in a second nodeof a network comprises receiving a channel estimation signal from afirst node; estimating characters of a channel between the first and thesecond node; determine a modulation type of each subcarrier for eachunicast profile based on the channel estimation; generating and sendingto the first node a channel estimation report indicating the determinedmodulation type of each subcarrier for each unicast profile.

According to an embodiment of the invention, a node in a networkcomprises a processing unit configured to compute a packet duration whenusing a first unicast profile with a first type preamble and a packetduration when using a second unicast profile with a second typepreamble, the processing unit is further configured to compare thecomputed packet durations so as to determine one from the first andsecond unicast profiles which yields a shorter duration; a transmittercoupled to the processing unit and configured to send the packet to theother node by using the determined unicast profile.

According to an embodiment of the invention, a node in a networkcomprises a receiver configured to receive a channel estimation signalfrom one other node; a processing unit configured to estimate charactersof a channel between the node and the other node, determine a modulationtype of each subcarrier for each unicast profile based on the channelestimation and generate a channel estimation report indicating thedetermined modulation type of each subcarrier for each unicast profile;a transmitter configured to send the generated channel estimation reportto the other node.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a block diagram illustrating a transmitter according to anembodiment of the invention;

FIG. 2 is a block diagram illustrating a receiver according to anembodiment of the invention;

FIG. 3 is a block diagram illustrating a channel estimator and equalizerfor a Type P6 preamble;

FIG. 4 is a block diagram illustrating a channel estimator and equalizerfor a Type P5 preamble;

FIG. 5 is a flowchart illustrating a method of selecting a unicasthigh-speed profile; and

FIG. 6-FIG. 23 are plots illustrating throughput enhancement;

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The following description is provided to enable any person havingordinary skill in the art to make and use the invention, and is providedin the context of a particular application and its requirements. Variousmodifications to the embodiments will be readily apparent to thoseskilled in the art, and the principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, the present invention is not intended tobe limited to the embodiments shown, but is to be accorded the widestscope consistent with the principles, features and teachings disclosedherein.

FIG. 1 is a block diagram illustrating a transmitter 100 according to anembodiment of the invention. The transmitter 100 comprises a Low-DensityParity Check (LDPC) error correction encoder 110 coupled to a subcarrierbit-mapper 120, which receives input from a modulation per Subcarriermemory 140. An appender 130 receives input from the bit-mapper 120 and apreamble generator 150. Output of the appender 130 goes to an OrthogonalFrequency Division Multiplexing (OFDM) modulator 160, which outputs to aFilter 170 and then a RF Up-Converter 180.

During operation of the transmitter 100, an input Media Access Control(MAC) frame is first encoded via the Low-Density Parity Check (LDPC)error correction encoder 110. Then, a resulting serial bit stream isassigned to subcarriers according to the modulation profile given in themodulation per subcarrier memory 140 and finally mapping the bitsassigned to each subcarrier to the appropriate Quadrature AmplitudeModulation (QAM) by the mapper 120. The preamble sequence generated bythe preamble generator 150 is appended to the beginning of each packetby the appender 130. The stream of complex numbers input by the appender130 to the OFDM modulator 160 are converted to time domain samples bycomputing an Inverse Fast Fourier Transform (IFFT) on a block of 512complex numbers. Cyclic prefix samples are inserted at the beginning ofeach OFDM symbol. The cyclic prefix samples are generated from the lastNumber of Cyclic Prefix (N_(CP)) samples of the IFFT output which arecopied and prepended to form one OFDM symbol. Then, the stream ofsamples is filtered by the filter 170 and up-converted to theappropriated RF frequency by the up-converter 180.

FIG. 2 is a block diagram illustrating a receiver 200 according to anembodiment of the invention. The receiver 200 comprises a RFdownconverter 210 coupled to an analog to digital converter (ADC) 220,which outputs to a OFDM demodulator 230 and a preamble processor 260,which also outputs to the demodulator 230. In an embodiment, the ADC 220comprises two ADCs, one for I and one for Q streams. The demodulator 230outputs to a subcarrier bit-demapper 240, which received input from amodulation per subcarrier memory 270. The demapper 240 outputs to a LDPCdecoder 250.

During operation of the receiver 200, the down-converter 210 filters anddown-converts a received RF to baseband. The signal is converted todigital samples via two ADCs 220 for the I and Q paths. First thepreamble processor 260 uses the preamble to adjust the signal level,detect the received signal, estimate and correct any carrier frequencyoffset that the received signal may have, and estimate the channel basedon the channel estimation (CE) portion of the preamble. The OFDMdemodulator 230 converts the received samples to frequency domain via aFast Fourier Transform (FFT). The complex samples of each subcarrier areequalized using the equalizer coefficients obtained from the channelestimation based on the CE. Then, the equalized complex samples areconverted to up to 10 values of Log-Likelihood Ratio (LLR) according tothe modulation profile per subcarrier obtained from the modulation persubcarrier memory 270. The LLR values are the soft values needed forLDCP decoding. The LDPC decoder corrects 250 any error that the receivedPHY frame may have and sends the decoded frame to the MAC block.

The modulation used on each subcarrier (i.e., the value of b, asdescribed further below) is specified in the modulation profile in thememory 270, also sometimes called the bit-loading table. This table isset during profiling for each pair of transmitting and receiving nodes.

The modulation profile contains 480 values of b, one for eachsubcarrier: b_(i) is the value at subcarrier i, where it represents thesize of the modulation in bits, running from 1 for Binary Phase SiftKeying (BPSK), 2 for Quadrature Phase Shift Keying (QPSK), 3 for8-(Quadrature Amplitude Modulation) QAM, through 10 for 1024-QAM. Forany subcarriers for which bi=0, no demapping is needed because no datawere loaded onto the subcarrier. There are 32 subcarriers that alwayscontain zeros. These are called unavailable subcarriers. Any additionalnulls in the modulation profile are called unused subcarriers.

The MOCA 2.0 standard defines two types of unicast high-speed profiles,one for NPER and another one for VLPER. The unicast profiles support amode of unicast profiles which can use either Type P5 preamble or TypeP6 preamble. Type P5 preamble uses one CE-symbol and Cyclic Prefix (CP)of 192 samples preceding it and Type P6 preamble uses two CE symbols andCyclic Prefix (CP) of 192 samples preceding them.

Conventionally, the MoCA 2.0 standard allows the use of only one NPERunicast profile; either Type P5 preamble, or Type P6 preamble, but notboth. Similarly, since the standard allows the use of only one VLPERunicast profile, it can use either Type P5 preamble, or Type P6preamble, but not both.

FIG. 3 and FIG. 4 are block diagrams illustrating a channel estimatorand equalizer for a Type P6 preamble and P5 preamble, respectively.

The channel estimation is obtained from the training sequences called CEwhich are part of the preamble. The standard channel estimation is doneas follows:

-   -   For Type P6 preamble, where two identical CE symbols are        available, the two CE symbols are first averaged after being        converted to the frequency domain        Y _(k)=(X _(1,k) +X _(2,k))/2  Equation 1    -   For Type P5 preamble, where only one CE symbol is available,        there is no CE symbols averaging. The received samples of the CE        are converted to the frequency domain obtaining        X _(k) , k=−N/2, . . . , N/2−1 Y _(k) =X _(k)  Equation 2    -   The least squares estimation of the channel frequency response        per subcarrier is given by

$\begin{matrix}{H_{k}^{LS} = \frac{Y_{k}}{s_{k}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

-   -   The channel frequency response estimate can be improved by using        the frequency domain correlation and the time domain correlation        of the channel frequency response. The improved channel        frequency response is generally a linear combination of the        least squares channel coefficients, H_(k) ^(LS). As is shown        later, the phase error of the channel coefficients due to the        residual CFO is fixed per subcarrier; thus, the improvement of        the channel estimate is not affected by the fixed phase error.        We will not address here the use of the correlation of the        channel response to improve the channel estimation.    -   The equalizer coefficients, Q_(k), per each subcarrier k are        estimated via

$\begin{matrix}{Q_{k} = \frac{1}{H_{k}^{LS}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

First the frequency domain tones (or subcarriers) of the received two CEsymbols are averaged by summing and dividing by 2. Then, the estimatedchannel frequency response per subcarrier is obtained by dividing eachY_(k) by the transmitted data of the CE symbol, s_(k), retrieved from amemory. The equalizer coefficients are obtained by inverting the channelcoefficients.

FIG. 5 is a flowchart illustrating a method 500 of selecting a unicasthigh-speed profile. This method 500 is compatible with the current MoCA2.0 standard specifications without modifications. However, the method500 requires the cooperation of two communicating nodes on the use ofthe unicast high-speed profiles. The NPER and VLPER unicast profilesthat are defined in the standard can be utilized. This method 500 usesjust one unicast profile for both NPER and VLPER packets because theperformance of NPER and VLPER unicast profiles is similar. So the legacyNPER can be used for NPER and VLPER unicast profiles using Type P5preamble, and the other legacy profile of VLPER unicast profile, whichis available in the MoCA 2.0 standard, can be used for NPER and VLPERunicast profiles using Type P6 preamble.

The standard defines an Error Vector Magnitude (EVM) probe which is usedto determine the optimum modulation per subcarrier. A transmit node,e.g., the transmitter 100, sends an EVM probe or a couple of EVM probesto a receiving node, e.g., the receiver 200. In an embodiment, the EVMProbe can be read from a memory in parallel to the LDPC encoder 110, andpass thru 120, the switch 130 to the OFDM modulator. At the beginning ofthe EVM probe, the preamble is sent first through the switch 130.

The receiving node uses the EVM probe(s) to estimate the EVM (orequivalently the signal-to-noise ratio (SNR)) per subcarrier. Based onthe EVM per subcarrier, a EVM processor (not shown) in the receivingnode derives the modulation per subcarrier for NPER and VLPER unicasthigh-speed profiles based on EVM or noise per subcarrier. The receivingnode sends the modulations per subcarrier to the transmitting node forNPER and VLPER high-speed unicast profiles via a Unicast BitloadingReport. The Unicast Bitloading Report includes also the preamble typethat must be used for transmissions using this profile.

In order to enhance the throughput, the receiving node can use a VendorSpecific Report, which defines packets that are used to send controlinformation or messages between two nodes, to send the modulations persubcarrier for NPER and VLPER high-speed unicast profiles using Type P5preamble using the current NPER unicast high-speed profile, and themodulations per subcarrier for NPER and VLPER profiles using Type P6preamble using the current VLPER unicast high-speed profile.

To keep the use of separate unicast high-speed profiles for NPER andVLPER, we can modify the MoCA 2.0 standard specifications by adding twonew high-speed profiles thereby yielding a total of 4 unicast high-speedprofiles. Two unicast high-speed profiles will support NPER packetswhere one profile will use Type P5 preamble and the other profile willuse Type P6 preamble. The other two unicast high-speed profiles willsupport VLPER, where one profile will use Type P5 preamble and the otherone Type P6 preamble.

The transmitter 100 sends a EVM Probe Report with the derivedmodulations per subcarrier for the 4 unicast high-speed profiles:

1. NPER Unicast high-speed Profile that uses Type P5 Preamble

2. NPER Unicast high-speed Profile that uses Type P6 Preamble

3. VLPER Unicast high-speed Profile that uses Type P5 Preamble

4. VLPER Unicast high-speed Profile that uses Type P6 Preamble

The channel estimation process at the receiver is based on the channelestimation (CE) sequence of the preamble. The equalizer coefficients arederived from conventional channel estimation for Type P5 and Type P6preambles. The equalizer coefficients based on Type P5 preamble may beless accurate than the ones based on the longer Type P6 preamble. Thelower quality of the equalizer coefficients reduces the performance byabout 3 dB. Therefore, in order to have similar performance, the unicasthigh-speed profile that uses Type P5 preamble should have lowermodulations per subcarrier relative to the unicast high-speed profilethat uses Type P6 preamble. Since the performance of the unicasthigh-speed profile using Type P5 preamble is reduced by about 3 dB dueto the equalizer coefficients quality, it's expected that themodulations of the subcarriers will be reduced by about 1 bit relativeto the unicast high-speed profile that uses Type P6 preamble. On onehand, the use of Type P5 preamble reduces the duration of the preambleand as a result it reduces the total size of the packet. However, on theother hand, the reductions of the modulations per subcarrier cause anincrease in the duration of the payload portion of the packet.

When a node gets a packet for transmission, a CPU or other relevanthardware, can select the unicast high-speed profile out of the twopossible unicast high-speed profiles that yields the shortertransmission duration. Then, the transmitter 100 of the node transmits abandwidth Reservation Request (RR) to the NC (Network Controller) thatincludes the information of the selected unicast high-speed profilenumber (or which unicast high-speed profile was selected). Note thatwithin the Reservation Request, the requesting node includes, amongother parameters, the selected modulation profile number and thetransmission duration. The transmitter node computes the expected packetlength using the two possible unicast high-speed profiles, where oneuses Type P5 preamble and the other one uses Type P6 preamble. Theprofile that achieves the shorter transmission duration is selected by anode's CPU or other relevant hardware, and is included in thereservation request message.

Method 500 comprises receiving (510) a new packet to transmit. Computing(520) a PHY payload. Then, simultaneously or sequentially calculating(530, 540) packet duration using P5 and P6 preambles. The durations arethen compared (550) and a bandwidth reservation request indicatingprofile with shorter duration is sent (560). The method 500 then ends.

FIG. 6-FIG. 23 are plots illustrating throughput enhancement usingembodiments of the inventions. For simplification, the throughputenhancement analysis assumes the following:

-   1) the cyclic prefix (CP) length is 128 samples which corresponds to    channel impulse response length of about 1 micro-second,-   2) the modulation of the unicast high-speed profile that uses Type    P6 preamble is higher by one bit relative to the modulation of the    unicast high-speed profile that uses Type P5 preamble,-   3) all the 480 subcarriers have the same modulation.

The throughput enhancement is calculated relative to conventionalprofiles:

-   1) the conventional unicast high-speed profile uses Type P5    preamble,-   2) the conventional unicast high-speed profile uses Type P6    preamble.

In FIG. 6-FIG. 14, the throughput enhancement of an embodiment iscompared to the conventional case where the unicast high-speed profileuses Type P5 preamble. Specifically, the throughput enhancement is shownfor different modulations as a function of the payload size in bytes.It's clear that when the packet is very long the effect of the preamblelength is negligible. However, as the packet duration is shorter, thelength of the preamble affects the total packet duration and thethroughput enhancement is reduced, as shown in the next plots. That is,the throughput enhancement diminishes as the packet length gets smaller.

The throughput enhancement is given by:

${Throughput\_ Enhancement} = \frac{\begin{matrix}{{{Packet\_ Length}\left( {{Type}\; P\; 5{Preamble}} \right)} -} \\{{Packet\_ Length}({ProposedScheme})}\end{matrix}}{{Packet\_ Length}({ProposedScheme})}$

The achievable maximum throughput enhancement reaches 100% for very longpackets and when the modulation of the proposed unicast high-speedprofile that uses Type P6 preamble is QPSK. Also it's assumed that inorder to keep the same performance, the modulation of the conventionalhigh-speed profile, which uses Type P5 preamble, is BPSK. The effect ofthe preamble length is negligible for very long packets. Therefore, thethroughput enhancement is proportional to the modulation ratio only. Thethroughput enhancement for very long packets that use the modulation ofQPSK relative to BPSK reaches 100%.

${Throughput\_ Enhancement} = {\frac{{QPSK}\left( {{bits}/{subcarrier}} \right)}{{BPSK}\left( {{bits}/{subcarrier}} \right)} = {\frac{2}{1} = 2}}$

The throughput enhancement diminishes as the profile uses highermodulation. The throughput enhancement for very long packets that usethe modulation of 1024-QAM relative to the conventional profile thatuses 512-QAM (1 bit reduction because it uses Type P5 preamble) reaches11%.

${Throughput\_ Enhancement} = {\frac{1024{{QAM}\left( {{bits}/{subcarrier}} \right)}}{{512{{QAM}\left( {{bits}/{subcarrier}} \right)}}\;} = {\frac{10}{9} = 1.11}}$

In FIG. 15-FIG. 23, the throughput enhancement of an embodiment iscompared relative to the conventional case where the unicast high-speedprofile uses Type P6 preamble.

The throughput enhancement is given by:

${Throughput\_ Enhancement} = \frac{\begin{matrix}{{{Packet\_ Length}\left( {{TypeP}\; 6\;{Preamble}} \right)} -} \\{{Packet\_ Length}({ProposedScheme})}\end{matrix}}{{Packet\_ Length}({ProposedScheme})}$

The maximum throughput enhancement reaches 38% for short packets wherethe payload occupies only one OFDM symbol.

The duration of a short packet where its payload occupies one OFDMsymbol, which uses the conventional unicast high-speed profile that usesType P6 preamble, is given by:Packet Length(P6)=CP ₀+2·CE+CP+OFDM_SymbolPacket Length(P6)=192+2·512+128+512=1856 Samples

The duration of a short packet where its payload occupies one OFDMsymbol, which uses an embodiment of the invention, will use the unicasthigh-speed profile that has Type P5 preamble because it's shorter thanthe packet that its payload occupies one OFDM symbol and uses Type P6preamble. Note that although the modulation of the payload of the packetthat uses Type P5 preamble is reduced by one, it's assumed that thepayload is adequately short and it still uses one OFDM symbol. Thepacket duration of the proposed scheme is given by:Packet Length(P5)=CP ₀ +CE+CP+OFDM_SymbolPacket Length(P5)=192+512+128+512=1344 Samples

Therefore, the throughput improvement for short packets reaches 38%:

${Throughput\_ Enhancement} = \frac{{{Packet}\mspace{14mu}{Length}\mspace{14mu}\left( {P\; 6} \right)} - {{Packet}\mspace{14mu}{Length}\mspace{14mu}\left( {P\; 5} \right)}}{{Packet}\mspace{14mu}{Length}\mspace{14mu}\left( {P\; 5} \right)}$$\mspace{20mu}{{Throughput\_ Enhancement} = {\frac{1856 - 1344}{1344} = 0.38}}$

However, for very long packets where the effect of the short Type P5preamble relative to Type P6 preamble diminishes, there is no throughputenhancement, as shown in the plots.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A method in a first node of a network,comprising: computing a packet duration when using a first unicastprofile with a first type preamble and a packet duration when using asecond unicast profile with a second type preamble; comparing thecomputed packet durations so as to determine one from the first andsecond unicast profiles which yields a shorter duration; sending thepacket to a second node by using the determined unicast profile; andgenerating and sending a resource request to a control node, theresource request notifying the determined unicast profile and theshorter packet duration determined through the comparison.
 2. The methodof claim 1, further comprising: sending a channel estimation signal tothe second node; and receiving a channel estimation report from thesecond node which indicates a modulation type for each subcarrier ofeach unicast profile, wherein sending the packet to the second nodefurther comprises sending the packet to the second node by further usingthe indicated modulation type for each subcarrier for the determinedunicast profile.
 3. The method of claim 1, wherein sending the packetfurther comprises sending the packet to the second node with resourcesgranted by the control node by using the determined unicast profile. 4.The method of claim 1, wherein the first preamble has a different lengthfrom the second preamble.
 5. The method of claim 1, wherein the firstunicast profile uses a higher modulation than the second unicast profileif the first preamble is longer than the second preamble, and the firstunicast profile uses a lower modulation than the second unicast profileif the first preamble is shorter than the second preamble.
 6. The methodof claim 1, wherein computing packet durations further comprises:computing a nominal packet error rate (NPER) packet duration when usinga first nominal packet error rate unicast profile with the first typepreamble and a nominal packet error rate packet duration when using asecond nominal packet error rate unicast profile with the second typepreamble; wherein the comparing further comprises: comparing thecomputed nominal packet error rate packet durations so as to determineone from the first and second nominal packet error rate unicast profileswhich yields a shorter duration; and computing packet durations furthercomprises: computing a very low packet error rate (VLPER) packetduration when using a first very low packet error rate unicast profilewith the first type preamble and a very low packet error rate packetduration when using a second nominal packet error rate unicast profilewith the second type preamble; wherein the comparing further comprises:comparing the computed very low packet error rate packet durations so asto determine one from the first and second very low packet error rateunicast profiles which yields a shorter duration.
 7. The method of claim6, further comprising: maintaining two unicast profiles, one of which isprovided with the first type preamble and used as both the first nominalpacket error rate unicast profile and the first very low packet errorrate unicast profile, the other of which is provided with the secondtype preamble and used as both the second nominal packet error rateunicast profile and the second very low packet error rate unicastprofile.
 8. The method of claim 6, further comprising: maintaining fourunicast profiles, a first one of which is provided with the first typepreamble and used as the first nominal packet error rate unicastprofile, a second one of which is provided with the second type preambleand used as the second nominal packet error rate unicast profile, athird one of which is provided with the first type preamble and used asthe first very low packet error rate unicast profile, and a fourth oneof which is provided with the second type preamble and used as thesecond very low packet error rate unicast profile.
 9. The method ofclaim 1, wherein Multimedia over Coax Alliance standard is employed insaid network.
 10. The method of claim 1, wherein each modulation typeincludes one of the following: BPSK, QPSK, 8-QAM, 16-QAM, 32-QAM,64-QAM, 128-QAM, 256-QAM, 512-QAM and 1024-QAM.
 11. The method of claim1, further comprising: receiving a channel estimation signal from athird node; estimating characters of a channel between the first and thethird nodes; determine a modulation type of each subcarrier for eachunicast profile based on the channel estimation; and generating andsending to the third node a channel estimation report indicating thedetermined modulation type of each subcarrier for each unicast profile.12. A method in a second node of a network, comprising: receiving achannel estimation signal from a first node; estimating characters of achannel between the first and the second node; determine a modulationtype of each subcarrier for each unicast profile based on the channelestimation; generating and sending to the first node a channelestimation report indicating the determined modulation type of eachsubcarrier for each unicast profile; and maintaining a plurality ofunicast profiles, a unicast profile being provided with a first orsecond type preamble and used as a nominal packet error rate unicastprofile, a very low packet error rate unicast profile, or both.
 13. Themethod of claim 11, wherein maintaining the plurality of unicastprofiles comprises: maintaining two unicast profiles, one of which isprovided with a first type preamble and used as both a first nominalpacket error rate unicast profile and a first very low packet error rateunicast profile, the other of which is provided with a second typepreamble and used as both a second nominal packet error rate unicastprofile and a second very low packet error rate unicast profile.
 14. Themethod of claim 11, wherein maintaining the plurality of unicastprofiles comprises: maintaining four unicast profiles, a first one ofwhich is provided with the first type preamble and used as the firstnominal packet error rate unicast profile, a second one of which isprovided with the second type preamble and used as the second nominalpacket error rate unicast profile, a third one of which is provided withthe first type preamble and used as the first very low packet error rateunicast profile, and a fourth one of which is provided with the secondtype preamble and used as the second very low packet error rate unicastprofile.
 15. A node in a network, comprising: a processing unitconfigured to compute a packet duration when using a first unicastprofile with a first type preamble and a packet duration when using asecond unicast profile with a second type preamble, wherein theprocessing unit is further configured to compare the computed packetdurations so as to determine one from the first and second unicastprofiles which yields a shorter duration; and a transmitter coupled tothe processing unit and configured to send the packet to the other nodeby using the determined unicast profile, wherein the processing unit isfurther configured to generate a resource request notifying thedetermined unicast profile and the shorter packet duration determinedthrough the comparison.
 16. The node of claim 15, wherein thetransmitter is further configured to send a channel estimation signal tothe other node; the node further comprises a receiver configured toreceive a channel estimation report from the other node which indicatesa modulation type for each subcarrier of each unicast profile; and thetransmitter is further configured to send the packet to the other nodeby further using the indicated modulation type for each subcarrier forthe determined unicast profile.
 17. The node of claim 15, wherein thetransmitter is further configured to send the generated resource requestto a control node, and send the packet to the other node with resourcesgranted by the control node by using the determined unicast profile. 18.The node of claim 15, wherein the first preamble has a different lengthfrom the second preamble.
 19. The node of claim 15, wherein the firstunicast profile uses a higher modulation than the second unicast profileif the first preamble is longer than the second preamble, and the firstunicast profile uses a lower modulation than the second unicast profileif the first preamble is shorter than the second preamble.
 20. The nodeof claim 15, wherein the processing unit is further configured tocompute a nominal packet error rate (NPER) packet duration when using afirst nominal packet error rate unicast profile with the first typepreamble and a nominal packet error rate packet duration when using asecond nominal packet error rate unicast profile with the second typepreamble, and compare the computed nominal packet error rate packetdurations so as to determine one from the first and second nominalpacket error rate unicast profiles which yields a shorter duration; andcompute a very low packet error rate (VLPER) packet duration when usinga first very low packet error rate unicast profile with the first typepreamble and a very low packet error rate packet duration when using asecond nominal packet error rate unicast profile with the second typepreamble; and compare the computed very low packet error rate packetdurations so as to determine one from the first and second very lowpacket error rate unicast profiles which yields a shorter duration. 21.The node of claim 20, further comprising a storage device configured tomaintain two unicast profiles, one of which is provided with the firsttype preamble and used as both the first nominal packet error rateunicast profile and the first very low packet error rate unicastprofile, the other of which is provided with the second type preambleand used as both the second nominal packet error rate unicast profileand the second very low packet error rate unicast profile.
 22. The nodeof claim 20, further comprising a storage device configured to maintainfour unicast profiles, a first one of which is provided with the firsttype preamble and used as the first nominal packet error rate unicastprofile, a second one of which is provided with the second type preambleand used as the second nominal packet error rate unicast profile, athird one of which is provided with the first type preamble and used asthe first very low packet error rate unicast profile, and a fourth oneof which is provided with the second type preamble and used as thesecond very low packet error rate unicast profile.
 23. The node of claim15, wherein Multimedia over Coax Alliance standard is employed in saidnetwork.
 24. The node of claim 15, wherein each modulation type includesone of the following: BPSK, QPSK, 8-QAM, 16-QAM, 32-QAM, 64-QAM,128-QAM, 256-QAM, 512-QAM and 1024-QAM.
 25. The node of claim 15,wherein the receiver is further configured to receive a channelestimation signal from one another node; the processor is furtherconfigured to estimate characters of a channel between the node and theanother node, determine a modulation type of each sub carrier for eachunicast profile based on the channel estimation, generate a channelestimation report indicating the determined modulation type of eachsubcarrier for each unicast profile; and the transmitter is furtherconfigured to send the generated channel estimation report to theanother node.
 26. A node in a network, comprising: a receiver configuredto receive a channel estimation signal from one other node; a processingunit configured to estimate characters of a channel between the node andthe other node, determine a modulation type of each subcarrier for eachunicast profile based on the channel estimation and generate a channelestimation report indicating the determined modulation type of eachsubcarrier for each unicast profile; a transmitter configured to sendthe generated channel estimation report to the other node; and a storagedevice configured to maintain a plurality of unicast profiles, a unicastprofile being provided with a first or second type preamble and used asa nominal packet error rate unicast profile, a very low packet errorrate unicast profile, or both.
 27. The node of claim 26, wherein thestorage device is configured to maintain two unicast profiles, one ofwhich is provided with a first type preamble and used as both a firstnominal packet error rate unicast profile and a first very low packeterror rate unicast profile, the other of which is provided with a secondtype preamble and used as both a second nominal packet error rateunicast profile and a second very low packet error rate unicast profile.28. The node of claim 26, wherein the storage device is configured tomaintain four unicast profiles, a first one of which is provided withthe first type preamble and used as the first nominal packet error rateunicast profile, a second one of which is provided with the second typepreamble and used as the second nominal packet error rate unicastprofile, a third one of which is provided with the first type preambleand used as the first very low packet error rate unicast profile, and afourth one of which is provided with the second type preamble and usedas the second very low packet error rate unicast profile.